This invention relates to a method of producing cast coated paper. More particularly, the invention relates to a rewet casting method of producing cast coated paper having very favorable printability and high gloss at a high casting speed.
Conventional methods of producing cast coated high gloss paper for printing include (1) a wet casting method for obtaining a high gloss finish on the surface of the coating layer by pressing the paper carrying a coating layer in a wet state against a heated polished drum surface, (2) a gel-casting method for obtaining a high gloss finish on the surface of the coating layer by pressing the paper carrying a gelled coating layer against a heated polished drum surface, and (3) a rewet casting method for obtaining a high gloss finish on the surface of the coating layer by drying or half-drying a wet coating layer carried on the paper, plasticizing the layer by a rewetting step, and then pressing the coated surface on the paper against a heated polished drum surface.
In all of these cast coating finishing methods, a coating layer in a plastic state is pressed against and dried in contact with a heated drum surface and then is released therefrom. The difference in the plastic state of the coating layer between these methods, however, influences the ease of operation. Each of these methods has particular problems in obtaining quality cast coated paper, as outlined below.
In the wet casting method, a low speed operation at a heated drum surface temperature of below 100.degree. C. is required because, at a temperature of above 100.degree. C. the coating liquid is boiled and the coating layer is damaged.
In the gel-coating method, the gelled state of the coating layer allows the use of a heated drum temperature above 100.degree. C. In practice, however, it is not possible to obtain a very high casting speed because a relatively large amount of water contained in the coating layer has to be transferred into the base paper when the paper is brought into contact with the surface of the heated drum at the press roll nip, and also because it becomes difficult to adjust the degree of gelling of the coating layer.
In the rewet casting method, it is possible to maintain the surface temperature of the heated drum in a range of 90.degree. to 180.degree. C. because the coating layer previously has been dried. However, the rewetting of the previously dried coating layer results in a far lower degree of plasticity of the coating layer than that of layers obtained by other methods. This results in uneven contact of the coating layer with the heated drum surface. Although in a lower speed operation a relatively uniform high-gloss coated paper can be obtained, an increase in operation speed markedly deteriorates the uniformity of the coating layer surface condition due to pinhold marks and non-uniform gloss.
In order to overcome this problem in the rewet casting, some approaches have been proposed including a method of preliminarily smoothing the coating layer by subjecting the same to sufficient supercalendering before rewetting, and a method of pressing the coating layer against the heated drum surface at a higher pressure. However, these methods sacrifice considerably the merits of the cast coated paper such as high bulkiness and high stiffness. Thus, in order to improve the uneven contact, a method of brushing the coated paper before rewetting has been proposed. This method can smooth the surface of the coating layer, but tends to densify said surface and to impede evaporation of moisture in the rewetted layer to such an extent as to hamper high speed cast finishing. Said densifying also retards drying of printing ink on cast coated papers, inducing ink set-off problems.
Furthermore, a rewet casting method, which is generally performed at a high temperature and a high pressure, tends to cause the migration of the binder in the coating layer towards the surface when a coating layer is reconstructed after rewetting. This tendency is intensified by increasing casting speed, so that the coating layer is weakened and this causes piling and other problems in the printing operations.